Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are essential bioactive lysolipid mediators that regulate a range of developmental and physiological processes and are emerging as targets for a new class of molecular therapeutics. In the cardiovascular system, LPA is poised to serve as a mediator of atherothrombotic disease: it is abundant in atherosclerotic plaque, present in the blood, produced by activated platelets, triggers phenotypic responses in smooth muscle cells, and potentiates platelet activation. Exposure of LPA during atherosclerotic plaque rupture/erosion has been proposed as a key modulator of platelet thrombosis. We report a novel pathway for regulation of platelet LPA responses that involves recruitment of the lysophospholipase D enzyme, autotaxin, responsible for LPA production to activated platelets. We have also found that LPA inhibits mouse platelet function, and that approximately 20% of human volunteers also lack a stimulatory response to LPA. We have evidence that this inhibitory LPA-signaling pathway confers vascular protection from thrombosis in mice and potentially from atherosclerotic disease in humans. We suggest that a therapeutic strategy directed at blocking stimulatory platelet LPA responses while promoting inhibitory ones could result in beneficial anti-thrombotic effects. Implementation of this strategy requires the molecular characterization of platelet LPA responses and proof of a role for circulating or locally produced LPA in regulating hemostasis and thrombosis. The broad goal of this proposal is to provide mechanistic insight essential for establishing LPA signaling as a viable anti-thrombotic target. We will take advantage of the differences in murine and human platelet LPA responses to identify the receptor(s) responsible for LPA signaling in mice and humans and then generate a transgenic mouse with platelet-specific expression of a stimulatory LPA receptor that phenocopies the human LPA response so that processes of relevance to human disease can be studied in a small animal mode. We are uniquely well-prepared to achieve these goals based on our expertise in bioactive lipid signaling and the tools we have amassed to study LPA in the vasculature. Completion of these studies promises to provide valuable insight into regulation of platelet function and may have exciting implications for the development of novel anti-thrombotic strategies that target LPA signaling.